Wavelength-Scanned Surface-Enhanced Resonance Raman Excitation Spectroscopy

We explore the correlation between localized surface plasmon resonance (LSPR) of triangular Ag nanoparticles, molecular resonance, and the surface-enhanced resonance Raman excitation profile of molecules adsorbed on these nanostructures. Ag nanoparticles with various LSPRs were fabricated with nanosphere lithography (NSL). A monolayer of tris(2,2′-bipyridine)-ruthenium(II) (Ru(bpy)32+) was introduced to the NSL Ag nanoparticles, and its effect on the LSPR was monitored by UV−vis spectroscopy. Wavelength-scanned surface-enhanced resonance Raman excitation spectroscopy (WS SERRES) profiles of Ru(bpy)32+ adsorbed on Ag nanoparticle arrays were measured for excitation wavelengths in the range of 400−500 nm. The WS SERRES profiles are correlated, both spatially and spectrally, with the corresponding LSPR spectra of the nanoparticle arrays and with the solution absorption spectrum of Ru(bpy)32+. The WS SERRES profile peak position depends on the relative spectral position of LSPR and the molecular resonance. Quasi-static electrodynamics modeling was applied to simulate the WS SERRES profiles, and the calculations are in agreement with the experimental results, demonstrating that the WS SERRES profiles involve multiplicative electromagnetic and resonance Raman enhancements.